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Zap of Electricity Creates Fluid Situation for Liquid - New Substance Shifts to Solid and Back With Flip of Switch

With frost warnings popping up across the country, Americans are getting their seasonal reminder that when temperatures get low enough, liquids become solid.

By Rick Weiss
Washington Post Staff Writer

But what if there were a way to turn a liquid into a tough solid at ordinary temperatures with a mere zap of electricity -- and then have that solid become liquid again, instantaneously, simply by shutting off the current?

Now scientists say they have created such a fluid: one that responds to an electrical field by immediately turning as tough as hard plastic and which just as quickly -- within a few thousandths of a second -- turns back into liquid when the power is turned off.

The fluid breaks new ground in the race to develop futuristic phase-changing substances like those often pictured in science fiction thrillers -- such as the morphing fluidic material that proves such an advantage to the mercurial villain, T-1000, in the movie "Terminator 2."

In a few years, scientists said, substances such as this one -- which has the fluidity of buttermilk but becomes thick as tofu in medium-strength electrical fields and hard as plastic in stronger ones -- could find their way into a wide range of applications, including shock absorbers that adjust their stiffness from moment to moment.

"Most vibration dampers today are passive, like the springs on your car that absorb the bumps in the road," said study leader Ping Sheng, a physicist at the Hong Kong University of Science and Technology.

Those systems offer a fixed amount of resistance and so do not work well when bumps are anything but average, Sheng said. But imagine a car with sensors that detect every jiggle, large and small, and then instantly translate those movements into electrical signals that thicken or thin the fluid in a shock absorber.

"Now the damper can be active. It responds to the environment," Sheng said. "This makes damping much more effective."

In fact, similar devices have recently begun to be commercialized in a few high-end automobiles.

But advocates of Sheng's approach say his type of fluid has some advantages over the others, which are generally slower to change from liquid to solid and which, at their best, get only as hard as rubber.

With its extreme adaptability, experts said, the new fluid and others like it in the development phase could end up not only in vibration dampers but also in a new generation of locks, valves, clutches or other devices in which there is a need for materials with variable fluidity.

"This appears to be a very impressive strength," conceded J. David Carlson, a physicist with Lord Corp. of Cary, N.C., which makes the competing types of hardening fluids used in some models of Cadillacs and Corvettes and under the seats of some well-equipped farm tractors.

But Carlson and others were quick to add that Lord Corp.'s fluids -- which use magnetic fields rather than electrical fields to make them turn solid -- have advantages over Sheng's. Ultimately, experts said, both types are likely to find niches in which they are best suited.

The new material, largely created by Sheng's co-worker Weijia Wen, is the latest to come out of the nascent field of nanotechnology, a hybrid discipline drawing on physics and engineering in which scientists build materials from small numbers of atoms.

The fluid consists of silicone oil in which are suspended countless tiny spheres made of barium and titanium -- each one less than a ten-millionth of an inch in diameter and each coated with an astonishingly thin film of urea about one hundred-millionth of an inch thick.

Nearly 200 of the spheres lined up in a row would just span the diameter of a human hair.

Scientists are still trying to understand the rules by which such particles behave. Because of their tiny scale, the particles are beholden not so much to the primary force affecting people and large objects -- gravity -- but to subtle electrical, chemical and quantum forces, which are of no consequence to creatures such as scientists but which dominate the dynamics of the atomic-scale world.

When Sheng's suspension is exposed to an electrical field -- easily generated with electrical wires -- the free-floating spheres take on positive and negative charges at positions equivalent to their north and south poles. That leads them to stack up in columns that are very difficult to break, turning the fluid into a hard solid.

The search for such "electrorheological" fluids has been going on for decades, but enthusiasm waned when it appeared that no liquid would ever get hard enough in an electric field to be of much use.

Interest shifted to "magnetorheological" materials, which change from liquid to solid when exposed to a magnetic field -- the same kind of field that makes iron filings line up on a piece of paper over a magnet.

Those materials can get about as hard as rubber, which makes them useful in a number of applications, including Lord Corp.'s automotive dampening devices. Several have also been installed in the foundations of skyscrapers in Japan and in a bridge in China where, in conjunction with sensors to detect sway, they make those structures resilient to wind and earthquakes.

But Sheng's substance, described in yesterday's online edition of the journal Nature Materials, is significantly tougher than any magnet-activated material.

Frank E. Filisko, a University of Michigan physicist who works with such fluids, said Sheng's material remains less than perfect. Ideally, it would be even thinner and more watery than it is now in the absence of a field. And it has yet to prove its usefulness in anything bigger than tiny devices a few millimeters in diameter.

"I'm hesitant to get too excited," he said. "This field has had its ups and downs."

But in the coming decade, he and Sheng predicted, materials that change phase in response to electrical signals will revolutionize engineering because they will make it possible to open and close valves, engage and disengage clutches and perform many other kinds of mechanical actions as quickly as an electrical switch can be turned on or off -- far faster than even the fastest mechanical or hydraulic systems can react.

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